State of the art efficient and cost effective LED drivers are not dimmable in a retrofit situation secondary to electronic transformers e.g. in the case of being dimmed using a standard TRIAC dimmer. In general, an LED driver is understood as comprising a power converter such as a switching regulator or a linear regulator for powering an LED or assembly of LEDs and a control unit arranged to control the power converter and/or the LED assembly. LED drivers generally are powered from a DC input source where dimming of the light (in response to a user interface action) is typically realised by adjusting the duty cycle of the LED or LEDs of the application. As such, conventional LED drivers are not suited for being powered by a voltage source such as provided by a standard TRIAC dimmer. The reason being that the waveform after an electronic dimmer can vary substantially. As such, the instantaneous voltage available as input to the LED driver may be momentarily insufficient to power the LED or LEDs of the lighting application. A normal halogen light will average out the power received and will not be induced to flicker, although even with halogens, the low output levels are cumbersome and flicker is seen in many cases.
When conventional LED drivers are powered by a voltage source such as provided by a standard TRIAC dimmer, light flicker may equally occur when the current required by the LED driver falls below a minimum value of the holding current of the TRIAC. A standard TRIAC design may e.g. require a holding current (after firing each 100 Hz cycle in a 50 Hz mains frequency example) of between 30-50 mA. In order to ensure the required holding current when a voltage needs to be provided to the LED driver, it has been proposed in literature to provide a load in parallel to the LED driver in order to ensure that the minimum holding current is being supplied by the TRIAC dimmer. Maintaining such a current (in order for the TRIAC to maintain its conducting state) may result in an important dissipation, adversely affecting the efficiency of the lighting application.
Often, state of the art LED drivers to be powered from e.g. a mains AC supply, apply a comparatively large input filter capacitance (over 1 to 10 uF). Such capacitance can e.g. be applied after a unit for EMI filtering and rectification and before a power converter, e.g. an efficient switching regulator. There are some significant draw backs to a large (over 0.1 uF) capacitance in this location. The capacitor's size, weight, cost, reduced life expectancy, and its negative impact on Power Factor Correction (PFC) all lead to serious drawbacks in existing driver designs when the capacitor is significantly over 0.1 uF.
Another drawback of existing solutions is that significant size, weight, cost, reduced life expectancy of PFC circuitry is prohibitive for applying low power direct mains current drivers.
A further drawback of existing solutions is that in many cases the 100 Hz line frequency is found in the light output. For certain people a 100 Hz frequency may easily lead to nausea. In addition, moving the lighting application and the eyes of an observer relative to each other may lead to flicker and/or stroboscopic effects at such comparatively low frequencies.
It is an object of the invention to at least partially eliminate at least one of the above-mentioned drawbacks or to at least provide a usable alternative.
It is an object of the present invention to provide i.a. an LED based lighting application that is better suited for dealing with a power source differing from a DC power source.